A car bomb has exploded near a Damascus mosque in a serious blow to a mooted three-day truce that was already struggling to take hold across Syria.

The explosion late on Friday is reported to have caused scores of casualties. It took place as the ceasefire called to mark the Islamic festival of Eid al-Adha was already faltering in parts of the country, with regime forces and rebel groups blaming each other for the violations.

Car bombs have been increasingly used by rebel groups, including the Islamist Jabhat al-Nusraf organisation, which has some ideological links to the global jihad movement. However, such groups have largely targeted security bases.

This bomb hit Daff al-Shouk square, where families were celebrating Eid, on the southern outskirts of the capital. No group had claimed responsibility for the blast by nightfall.

Earlier, the truce brokered by the UN-Arab League peace envoy, Lakhdar Brahimi, had done little to stem the violence in parts of Syria, including parts of Damascus and ravaged districts of Aleppo.

The Local Co-ordination Committees, which monitor violence daily, reported 110 breaches of the truce and around 48 deaths, a toll that is sharply down on any other day in recent months. Other areas of the country, such as the cities of Homs and Hama, were quieter than in recent months. Activists there reported that families had taken to the streets of some neighbourhoods to celebrate one of the holiest occasions in the Islamic calendar.

In Aleppo, the centre of violence since rebels stormed the city in mid-July, gains made by opposition fighters in three districts on Thursday were partially clawed back by the army. Rebels had moved into the districts, reporting little resistance from a Kurdish militia in one of the areas and Syrian troops in two others. Little ground had previously changed hands in Aleppo despite more than two months of fighting. Syrian forces retain control of much of the west of the city and strategic points on the outskirts, such as the airport. However, they remain unable to secure the east of the city, despite near constant aerial bombardment of rebels in some districts.

On Thursday Turkey denied a Human Rights Watch claim that it was preventing thousands of Syrian refugees from crossing into its territory. Turkey is hosting about 103,000 Syrian refugees and says it expects thousands more as winter closes in. Refugee groups claim some of those fleeing have been unable to cross from Syria since late August. Lebanon and Jordan are sheltering similar numbers to Turkey. All three states say they are under-prepared for an expected influx in the last weeks of autumn.